Along with the recent intensification of waste water-control to cope with the environmental pollution problem, there has been an outstanding tendency in the metal-working industry for companies to exclude to metal surface treatment, which requires complicated treatment of waste water, from their own process, and utilize metallic materials which have already undergone surface treatment. The surface-treated steel sheets are representative of such metallic materials, and their production is smoothly increasing. This is an exception of the iron and steel products which suffer from a difficult situation in the trends of decreasing the material consumption by, for example, lightening, thinning, shortening and size-reducing of articles.
The properties, with which the surface-treated steel sheet must be provided, are various, depending upon their application such as: corrosion-resistance; adherence of paint film; post-painting corrosion-resistance; weldability; press formability; resistance against the fingerprint adhesion. Since the characteristic of the surface-treated steel sheet resides in that it is blank material for working, it is necessary to die-form them with the use of an oil agent. The result is that it must ordinarily undergo the degreasing process by aqueous alkaline solution. Therefore, it is important in practice that the above mentioned properties be provided after the alkali-degreasing.
Among the surface treated steel sheets zinc-plated steel sheets are subjected to a rust-proof treatment by chromating. However, since a satisfactory paint-adherence is not obtained by the chromate film alone, the sealing by organic resin is carried out to supplement the paint-adherence.
Japanese Unexamined Patent Publication No. 58-224175 discloses surface-treated steel sheets having an improved, post-degreasing, adherence of paint film. They utilize zinc-plated steel sheets as the blank material, which are first subjected to a reactive chromating treatment, then water-rinsing, and, finally, sealing with organic resin. According to the explanation by this publication, water-soluble acrylic resin added to the reactive chromating treatment liquid prevents peeling between the chromate film and the sealing film and is hence effective for improving the post alkali-degreasing properties. The surface-treated steel sheets produced by this process have improved, post alkal-degreasing-properteis, and, therefore, their application expands particularly in the makers of household electric appliances and it follows that the amount used by them steadily increase.
Nevertheless, since the above described process consists of two treating steps, i.e., the chromating and sealing of water-based resin, it is difficult to readily apply such process for production in the existing line for single step chromating treatment. Even if the application is possible, not only is reconstruction in a large scale necessary, but also the water-rinsing step becomes necessary because the reactive chromating treatment is carried out as the first chromating treatment. Consequently, the chromating treating liquid adhered on a plated steel sheet is brought into the water-rinsing step, and, therefore, the treatment of waste water, which contains hexavalent chromium becomes indispensable. Accordingly, the present inventors decided to further consider the coating of a chromate-containing and water-based resin, which can make possible a single step-treatment.
Heretofore, as is proposed for example in Japanese Examined Patent Publication No. 59-30,788, it has been considered that the chromating treating liquid and organic resin are applied on a steel sheet and the resin is polymerized by electron beam and the like. This however causes a problem because a special apparatus for the electron-beam irradiation and the like becomes necessary. Accordingly, the present inventors considered the coating of a chromate-containing and water-based resin which is based on the premise that an ordinary heating and drying furnace is used. Incidentally, the drying furnace is ordinarily a direct or indirect heating type, air-blast furnace and is referred to as the optimum for vaporizing the moisture. There has therefore been demands for developing the coating of a chromate-containing and water-based resin which is based on the premise that the air-blast drying furnace is used.
Incidentally, the sheet conveying speed in the production line of surface-treated steel sheets is usually from approximately 100 to 150 m/min. The zone length which is allowed for the treatment is usually approximately 10 m, and, therefore, the time required for drying must be around 5 seconds. In order to facilitate drying in such a short time, the steel sheets and coating liquid need to be heated up to around 40.degree. C. If temperature of the steel sheet and coating liquid is only around 30.degree. C. or lower, the liquid, which is uniformly applied on the steel sheet, does not vaporize due to its low equilibrium vapor pressure, but is rather moved by the hot air blast. The result is that the appearance of coating film after drying becomes non-uniform, and, hence, not only is the commercial value lost, but also its properties, such as the corrosion-resistance, are degraded.
The conventional, chromate-containing and water-based resin coating composition tends to gel upon the temperature-rise exceeding 30.degree. C. The problems hence arise that the above mentioned properties are lowered, and, in addition, the properties of paint-film are not constant. These problems can be avoided by heating the steel sheets and then applying the coating liquid having room temperature. This however incurs a new problem due to gel as follows. The surface treating line for the steel sheet is usually not provided with special roll-coater apparatus as is in the line of colored zinc-plated steel sheet, but is frequently provided with grooved rolls. The liquid is therefore brought into contact with a steel sheet under the natural coating. The temperature of liquid is therefore elevated, and this liquid is again reverted to a circulation tank. This results in the temperature of chromate liquid in the circulation tank rising to a temperature close to sheet temperature. Accordingly, the coating liquid needs to be stable in a certain period under the warmed state up to approximately 40.degree. C.
Although the above explanations are made mainly with regard to the surface-treated steel sheets, the present invention can be applied for similar surface treatments provided that similar problems are involved.
Heretofore, it is recognized that the acrylic resin exhibits improved adherence with regard metal and top coating paint. There is therefore a proposal (Japanese Unexamined Patent Publication No. 60-228,682) that the acrylic resin be used in the chromating treating liquid of zinc-plated steel sheet. However, since the emulsion of acrylic resin is subjected to the cohesion action and oxidation action by chromic acid which is a strong acid and a strong oxidizing agent, there is a problem involved in the miscibility with the aqueous solution of chromic acid.